The gain, which brings its user base to 255 million, represents a 55% acceleration in the rate of user growth versus the fourth quarter of 2013, when Twitter added only 9 million users despite being in the news almost continually in October and November thanks to its IPO.

Looking at year-over-year comparisons, on the other hand, the first quarter was a slowdown, with a gain of 25% versus 30.3% for Q4. That's probably why shares are down sharply in after-hours trading.

User growth has been the big question mark hanging over Twitter's head, as monetization has been relatively robust. That trend continued in the first quarter, with Twitter generating $250 million in revenue (80% of it from mobile), up 119% year over year. That beat the consensus Wall Street estimate (as tracked by Seeking Alpha) by $9.6 million, and the company also outperformed expectations by coming in at breakeven (on a non-GAAP basis) with earnings per share of 0.0 cents.

One bright sign is in international, where revenue grew 183% year-over-year. That's faster than the rate of growth for North America. Still, the rest of the world only accounted for 28% of Twitter's revenues in the quarter despite comprising nearly three-fourths of its user base.

Twitter also raised its outlook for the rest of the year, forecasting revenue of $270 million to $280 million for the second quarter and $1.2 billion to $1.25 billion for the full year.